The study of "Maternal Phenylketonuria" encompasses two major components: I. The mediated transport of L-phenylalanine and L-tyrosine and the possible competitive inhibition mechanism of aromatic amino acids at the placental level will be studied in pregnant rhesus monkeys. These primates will undergo an operative procedure whereby a polyvinyl catheter will be placed into the common carotid artery of the fetus which then will facilitate blood sampling (after maternal L-phenylalanine loading) throughout the remaining gestational period. II. "Phenylketonuria" will be induced in newborn female rhesus monkeys according to techniques described by Waisman & Harlow and Berry & Butcher (modified procedure). As these "phenylketonuric" monkeys grow up, various developmental parameters will be studied at specified time intervals. After maturing and successful mating, the monkeys will be fed varying high phenylalanine diets and their offspring will undergo periodic physical examinations and psychological assessments. The primary objective of this investigation will be the determination of an optimal serum phenylalanine concentration in the pregnant "phenylketonuric" primate, hence providing a favorable biochemical intrauterine milieu essential for a "normal" development of the fetus.